MCI MC9 air dryer

[solodon]

As I understand it, the air dryer purges air (the hissing sound) at about 120 psi.  Also there is a small line of some nature that comes from the compressor to the dryer and front air system.  When my air purges the pressure gauge immediately starts to drop and in a few minutes is down to 90 where it starts to climb again to 120.  This takes a couple of minutes, the air purges, and the cycle starts over again.  Is this normal?  If it's not normal and there is a leak somewhere where would be a good place to start looking.  Also if there is a leak would this explain the front bags going down in 30 minutes?  TIA  Don

[eb99603]

Hey Don. I'm no expert (by any means), but I've experienced some of the same problems you're encountering now.

If the bags drop in 30 minutes after shutting off the bus, yeah you have a leak. If it's *just* the front bags that empty, focus your initial search at the bags, leveling valves, fittings, etc associated with the front suspension. If *everything* or several things lose air in 30 minutes you have a much larger leak, and perhaps some bad check valves.

With respect to your constant compressor cycling, the short answer is no. It's not normal.

Check out this simplified diagram of the main components of your air system: http://www.gonefcon.com/trucktcom/air_dryer_system.gif The air compressor runs all the time. Mine's gear driven, connected to the engine. Yours probably is to (might be belt driven, I guess and I'm not up on all of the different coach engines and their differences). Anyway, it runs constantly. But at some point it needs to be told "hey man, stop filling the tanks with air" lest your pressure continue to grow and eventually...boom. This is where the governor comes into play. The governor's purpose is simple: at a particular pressure (~120psi), redirect some portion of the air to an outlet port. When the pressure drops below a certain point (~90psi), stop redirecting the flow. You can see in the picture I linked above, that there's an UNL port on the governor. When you reach 120psi, air's allowed out that port and to 1) the air compressor and 2) the dryer.

Two things happen at this point. First, the air compressor stops compressing air. The pistons are still moving, but instead of compressing air and filling your tanks with it, it's just going straight back out to atmosphere. The compressor is unloaded, and is effectively doing no work. Second, the dryer now has pressure feeding its unloader port. This results in the dryer purging air/moisture and the desiccant is allowed time to dry.

If you're dropping pressure when the governor kicks out (at ~120 psi) you're likely bleeding air at something connected to the unloader port of your governor. It might be a fitting somewhere. But more than likely it's either a) the unloader on the compressor itself. This was my problem and in my case I bought an unloader repair kit and replaced the unloader pistons, springs, and o-rings which stopped my air leak. Took about 20 minutes. Or b) you have a bad check valve at the dryer. When the dryer purges the check valve is supposed to prevent the rest of your tank air from being expelled out the same port that the wet air is being purged. But if that check valve's shot, you'll bleed air out the dryer. In either case you bleed air right up until the point where the pressure drops low enough that the UNL (unloader) port is no longer pressurized and the purge valve is closed. Then you start building air again.

Hope that helps.

[Jim Eh.]

Soapy water in a spray bottle is your best friend....

The two systems are somewhat isolated and may each have leaks in them to cause your condition. Does the air system continue to drop below 90 if the bus is shut off?

[solodon]

Thanks for the link.  I'll check it out and start the investigation.  Did the rear bags and valves last year and figured I'd need to do the front also.  However after reading some stuff on here recently the cycling had be bothered.  Thanks for the help.

Yes the air system (on the gauge) will drop to 0 within that same half hour to an hour.  The rear stays up for days.  The front drops fairly quick.

[eb99603]

Sounds like you have a hefty leak in the front suspension. Address that first and then see if you still drop air pressure when the governor kicks out. If it still does, then move on to the unloader and drier check valve.

Good luck!

[Jim Eh.]

Actually even if your air ride has a significant leak, your system pressure should not drop below 60 psi. There is (or once was) a pressure protection valve that closes to isolate and make available the remaining system pressure for brake applications, in case of a catastrophic air loss such as a blown air bag. I should have asked if your air system drops below 60

If your system pressure drops to zero and your front suspension goes down, then you have more than one leak to look for. but to keep things simple work on one at a time as suggested. Just keep in mind they are probably not related to each other.

[Dave5Cs]

Also depends on where your gauge is feed from accessory dry tank etc. Might also replace your f2f governor for less than 13 dollars napa and rebuild your air dryer or replace cartridge.

[solodon]

f2f governor???

[Dave5Cs]

Sorry D2

[Dave5Cs]

Sorry D2

Finger slipped. In a bus in Jamaica going to resort . This guy drives like a fool on cliff side of ocean.

[solodon]

Sorry for the ignorance.  You meant D2?  D2 is what?

[luvrbus]

The D-2 is a unloader it keeps the pressure between max and min in the reservoir tanks from the compressor @ a set pressure but I don't think that is your problem

[eb99603]

I don't want to dwell on semantics, but it seems Don's relatively new to the air system and I want to make sure the terminology's right.

The D2 luvrbus is referring to is a Bendix D2 Governor. Here's the manual for it: http://www.manualslib.com/manual/392610/Bendix-D-2-Governor.html It's not technically an unloader - the unloader exists in the compressor and is what actually causes the compressor to stop compressing air. A governor, on the other hand determines when you would like to unload. As luvrbus mentioned, the governor has max/min pressures (cut out/in pressures). Once the max pressure is reached, it trips the unloader in the compressor and air pressure stops building. As you use air and the pressure drops you will eventually get lower than the min pressure, at which point the governor stops sending pressure to the unloader and thus the compressor starts building pressure again.

Refer to the simple diagram I linked above. Air goes into the governor, and when the system pressure reaches the max point, air is directed to the governor's unloader port which goes to the compressor unloader, and to the drier's unloader port (which allows the drier to purge the air).

It's probably worth noting that the governors come with different factory-set min/max pressures and are (typically) also adjustable. Lastly, I agree with luvrbus - the governor (which is probably a D2 model on your bus) could be the problem, but it's probably not.

[Jim Eh.]

Does your air dryer continue to hiss air to when the system reaches 90 psi after the initial blast @ 120 psi?

[eb99603]

Hey Don. Did you have a chance to spend any quality time with a bottle of soapy water and your bus this weekend?